(i) Field of the Invention
The present invention relates to molded articles of a polypropylene which are transparent and excellent in impact resistance. More specifically, it relates to transparent and impact-resistant molded articles obtained by blow molding a polypropylene having a substantially syndiotactic structure, or a composition comprising a polypropylene having a substantially syndiotactic structure and a copolymer of ethylene and propylene as well as transparent multi-layer molded articles having good impact-resistance which are obtained from the above-mentioned composition and in which at least one surface of each article has a polypropylene layer having a substantially syndiotactic structure.
(ii) Description of the Prior Art
A syndiotactic polypropylene has been known for a long period of time. However, a polypropylene obtained by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound is poor in syndiotacticity. Therefore, the thus prepared polypropylene is hardly considered to have characteristics of the syndiotactic polypropylene, and much less, a copolymer of ethylene and propylene which can be obtained by the use of the above-mentioned catalyst is scarcely regarded as a crystalline polypropylene.
On the contrary, a polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 has been discovered for the first time by J. A. Ewen et al. which can be obtained by the use of a catalyst comprising a transition metal compound having asymmetric ligands and an aluminoxane (J. Am. Chem. Soc., 110, 6255-6256, 1988).
Molded articles which can be prepared by molding a syndiotactic polypropylene obtained by the above-mentioned J. A. Ewen et al. method have good transparency and relatively excellent stiffness, but they have the problem that their impact resistance at low temperatures is bad.
On the other hand, an isotactic polypropylene is inexpensive and good in heat resistance, and therefore it can be blow molded to manufacture bottles and the like.
However, molded articles made from the isotactic polypropylene have good heat resistance, but they are poor in transparency and bad in impact resistance at low temperatures. Thus, for the purpose of improving the impact resistance at the low temperatures, a block copolymer with ethylene can be used, but in this case, there is the problem that its transparency is impaired.